1. Field of the Invention
Economical availability of oil and gas from geological reservoirs has been and remains of immeasurable importance. Such reservoirs vary in accessibility from wells of a few hundred feet to over two miles in depth.
In order to attain fuller recovery in a given time from what is otherwise available, a technique of fracturing the rock in underground formations has been practiced since at least as early as 1949. See the U.S. Pat. No. 2,596,843, to Farris, and the publications, Parts 1 and 2, entitled "Overview of Current Hydraulic Fracturing Design and Treatment Technology" by R. W. Veatch, Jr., from the April and May 1983 issues of Journal of Petroleum Technology, pages 677-687 and 853-864. The latter discusses hydraulic fracturing techniques, characteristics and surveys of fractures based on temperature, pressure, radioactivity, and acoustical data.
Such fracturing involves pumping fluid into a well at a selected level at sufficient pressure to produce separation of portions of the formation. This typically results in some leak-off of the fluid and a reduction in the pressure level. The fracture may usually be extended by continued pumping of fluid into it.
In order to enhance oil or gas recovery, a granular substance such as silica sand is pumped into the crevice to prop the portions apart upon the decline of fluid pressure.
Since the fracture treatment is directed at levels viewed as likely to contain reservoirs, it is often carried out at deep depths, as mentioned above.
Such fracture treatment involves large outlays for equipment and pumping fluid and must, therefore, be tailored to suit each individual environment. Furthermore, in a given area, there may be a multiplicity of wells whose spatial and other relationship requires a particular fracture design.
Attempts to determine the specific nature of fractured formations have been made through various procedures in the past in order to enable the designer to learn the results of fracture designs and to produce fracturing with predictable results. Once a hydraulic fracture treatment has been performed, delineating the actual location (depth to the top) and vertical extent (height) of the affected zone is important. The actual fracture that occurs may differ substantially from the designed fracture. For example, fractures can break through natural geologic barriers which were anticipated to contain the fracture. Fractures may grow down when predicted to grow up and out. In some cases, no reliable pretreatment prediction can be made.
Hence, independent confirmation or delineation of fracture depth and height is needed. Such data is then used for (a) evaluating the success or failure of the treatment design, (b) making recommendations about future treatments, (c) successfully operating the well, and (d) successfully engineering the surrounding reservoir. In addition, when combined with other data types, the data can be important in making decisions about drilling in new fields and the legal requirements for drilling in existing fields.
Because of the isolated and inaccessible nature of fracture formations, and the seeking of oil and gas at greater depths with the accompanying increased temperature levels and lower permeability, the need and difficulty of applying appropriate fracture treatments has increased significantly.
Fracture survey methods used in the past have included the sensing of fluid flow, radioactivity, temperature, pressure, including pulses, and resonance; and the seismic and acoustic sensing within a bore and on the surface of the ground adjacent to a bore of motion or microseismic activity or signals generated from sources outside of the fracture formation. Such methods have included analysis of the data by various methods and the use of theoretical computer models.
However, for various reasons, the past survey and analysis methods have not been reliable, resulting in the loss of product potential and the uneconomical use of fracturing resources. Among the reasons for the unreliability of past methods has been the migration of fluid in the well bore out of the region being sensed, resulting in inaccurate fluid property data. Other methods have assumed a single plane fracture, contrary to fact. Various methods have made incorrect assumptions in their analysis of the data received and hence have failed to obtain valid results. Thus, there has been an unfulfilled need for a reliable fracture survey and analysis method.
2. Description of the Related Art
Illustrative patents and publications description of the related art include the following.
The U.S. Pat. Nos. 2,951,535, 3,306,102, 3,332,483, 3,402,769, to Mihram et al. Lebourg, Wyllie, and Doggett et al. disclose the use of radioactive substances in fracturing fluids which are detected and logged in the bore hole in order to provide an indication of the location of a fracture.
The U.S. Pat. No. 3,205,941 to Walker discloses a logging tool in the well bore which generates and receives acoustic signals by reflection and refraction from discontinuities, including fractures.
The U.S. Pat. No. 4,057,780 to Shuck discloses detonating liquid explosives in fractures and monitoring the emissions with acoustic sensors at spaced locations above the well bore to determine the configuration and orientation of the fractures.
The U.S. Pat. No. 3,356,177 to Loren discloses transmitting acoustic impulses in a well in a manner to indicate acoustic wave interference and anomalies indicating fracture of the wall of the well.
The U.S. Pat. No. 4,310,346 to MacDonald and U.S. Pat. No. 4,328,567 to Dodge are further illustrative of patents disclosing the acoustical logging of a bore hole of a well.
The U.S. Pat. No. 3,427,652 to Seay discloses applying oscillating fluid pressure within a well zone subject to fracture and, following the application of such pressure, measuring the resonant frequency of the fluid oscillation, and repeating this procedure to obtain information respecting the fracture.
The U.S. Pat. No. 4,458,245 to Crosnier et al. discloses a sonde mechanism for pulsing fluid and sensing pressures and resonances in an isolated section of a well bore in order to determine fracture characteristics.
The U.S. Pat. No. 3,586,105 to Johnson discloses applying pulses of pressure within one well and sensing and analyzing the pressure changes in adjacent wells in order to determine vertical fracture orientation and other characteristics.
The U.S. Pat. No. 4,749,038 to Shelley discloses pumping fluid into a well, shutting in the well and monitoring the pressure in the well to determine the time required for predetermined change in pressure to occur in order to design a fracture treatment.
The U.S. Pat. No. 4,432,078 to Silverman discloses the generating of pressure pulses at the lower portion of a well and sensing these pulses in spaced relation around the top of the well on the surface in order to determine the azimuth of a fracture.
The U.S. Pat. No. 3,739,871 to Bailey discloses applying pressure in a well to cause fracturing, and sensing and recording the time of arrival of seismic waves on the surface of the earth at spaced locations around the well bore.
The U.S. Pat. No. 4,280,200 to Silverman discloses the creating of a seismic wave at the surface and over the supposed position of a fracture and detecting the seismic wave reflected upwardly at or near the fracture at each of a plurality of seismic sensors. The U.S. Pat. No. 4,524,434 to Silverman is of generally similar nature.
The U.S. Pat. No. 4,420,975 to Nagel et al. discloses the injecting of a fluid into a well bore that invades the earth formation and measuring or logging at different points in time a characteristic of the fluid such as resistivity or the decay time of thermal neutrons.
The U.S. Pat. No. 4,42,895 to Lagus et al. discloses applying fluid pressure within an isolated region of a bore hole isolated by packers and simultaneously monitoring pressure and flow in the region and adjacent regions in order to determine fracture characteristics.
The U.S. Pat. No. 4,109,717 to Cooke discloses the use of a rotatable temperature probe in a well for the purpose of sensing the relatively cool liquid in a fracture in order to determine the orientation of such fractures.
Analysis of the pressure decline following fracturing and while the well is shut-in, for the purpose of determining the volume of fracturing fluid required for extending a fracture, is described in the U.S. Pat. No. 4,398,416 to Nolte.
The U.S. Pat. No. 4,440,020 to Boutemy et al. discloses the making of a plurality of well logs at common depth intervals to find consistency between the logs in order to provide an indication of the geologic formation.
The U.S. Pat. No. 4,638,254 to Uhri discloses the analysis of data obtained from orientation logging in which unit vectors are employed to produce a resultant vector for indicating the orientation of the formation under study.
The 1986 publication SPE 15216 entitled "Advances in the Microseismic Method of Hydraulic Fracture Azmith Estimation" by Sorrells and Mulcahy discloses the monitoring from a nearby well by seismic sensors, pressure gauges, temperature probes and acoustic sensors for detecting high frequency pressure deviations, in order to determine the azimuth of a hydraulic fracture.
Prior to making the present invention, I was aware of the commercial employment of a sonde for sensing motion in a well immediately after a fluid pressurization phase for the purpose of determining the azimuth or direction of the hydraulic fracture. This was performed at several levels in order to reduce the possibility of error due to the effect of sonde deployment. However, the number of such levels was not related to the height of the fracture, nor for the purpose of determining it and, in fact, was insufficient for such purpose.
Furthermore, in 1986, I participated in an experiment seeking to determine the overall dimensions of a hydraulic fracture by detecting in a well bore an artificial wave energy source such as a mechanical, controlled-frequency band sweep device, a weight drop, an explosive, or a land air gun, applied at the earth's surface. The sonde detector was employed at different depths and its data was analyzed by me, for resonance and to compare the combined horizontal and vertical components of motion, in an attempt to indicate the fracture dimensions. This was unsuccessful.
I am aware of earlier patents such as the U.S. Pat. No. 1,909,205, to McCollum dated May 16, 1933, which describes the use of a seismic detector in a well bore at a plurality of locations for the purpose of analyzing the travel time of direct and reflected energy from a buried explosive charge or a dropped weight on the earth's surface. This is for the purpose of identifying and delineating pre-existing geological structures, whereas the present invention is concerned with determining the dimensions of a man-made hydraulic fracture.